Method and apparatus for time-division plasma chopping in a multi-channel plasma processing equipment

ABSTRACT

A multi-switch processing methodology and a multi-channel time-division plasma chopping device (10) for in-situ plasma-assisted semiconductor wafer processing associated with a plasma and/or photochemical processing equipment. The device (10) comprises a main transfer channel (72) associated with the processing reactor for transferring process gas and activated plasma mixtures into the reactor. A plurality of gas discharge channels (18, 22, 26, and 30) associate with the main transfer channel (72) for independently directing various gases and activated plasma combinations to main transfer channel (72). Process excitation sources (16, 20, 24 and 28) associate with at least one of said gas discharge or activation channels to independently and selectively activate process gases and to control gas activation and flow from the discharge channels to the main transfer channel (72). The method of the present invention performs multi-channel time-division plasma chopping by independently and selectively generating plasma or activated species using a plurality of remote plasma generation process energy sources (16, 20, 24, and 28) associated with the semiconductor wafer fabrication reactor.

This is a division of application Ser. No. 07/580,986, filed Sep. 12,1990; now U.S. Pat. No. 5,273,609.

TECHNICAL FIELD OF THE INVENTION

The present invention relates in general to semiconductor devicefabrication, and more particularly to a method and apparatus fortime-division plasma chopping and multi-switch semiconductor waferprocessing in a multi-channel plasma processing reactor.

BACKGROUND OF THE INVENTION

Manufacturers of microelectronic devices use a variety of waferprocessing techniques to fabricate semiconductor integrated circuits.One technique that has many important applications is known as plasmaprocessing. In plasma processing, a substantially ionized gas, usuallyproduced by a radio-frequency (RF) or microwave electromagnetic gasdischarge, provides activated neutral and ionic species that chemicallyreact to deposit or to etch material layers on semiconductor wafers inplasma processing equipment.

Applications of plasma-assisted processing techniques for semiconductordevice manufacturing include reactive-ion etching (RIE) of polysilicon,aluminum, oxide and polyimides; plasma-enhanced chemical-vapordeposition (PECVD) of dielectrics, metals, and other materials;low-temperature dielectric chemical-vapor deposition for planarizedinter-level dielectric formation; and low-temperature growth ofepitaxial semiconductor layers. Additional applications of plasmaprocessing include plasma surface cleaning and physical-vapor deposition(PVD) of various material layers.

In plasma-assisted deposition and etching, a process "activation switch"acts on the electromagnetic process energy source and begins theplasma-assisted fabrication process. Although not necessarily a tangible"ON/OFF" switch, an activation switch performs the process on/offfunction by starting a fabrication process by its activation of aprocess energy source (e.g., RF power source) and stopping the processby its removal of the energy source. An activation switch for aplasma-assisted process may, for example, be the presence of aradio-frequency electromagnetic gas discharge source or some otherprocess energy source without which the process cannot occur.

Semiconductor device fabrication processes including etch and depositiontechniques use a process activation switch in order to drive the desiredprocess. In plasma processing, the particular type of plasma-assistedprocess to be performed affects the process activation switch choice.The choice of activation switch for any device fabrication process,regardless of whether the process is a deposition or etch process, alsomay significantly affect the final semiconductor device properties.

For example, J. Gibbons et al., "Limited Reaction Processing: SiliconEpitaxy," Applied Physic Letters, 47(7), 1 Oct. 1985, pp. 721-23,describes a material layer deposition process known as limited-reactionprocessing or "LRP" that uses a thermal process activation switch (wafertemperature). On the other hand, K. Tanjimoto, et a l., "A New Side WallProtection Technique in Microwave Plasma Etching Using a ChoppingMethod," Extended Abstracts of the 18th (1986 International) Conferenceon Solid State Devices and Materials, Tokyo, 1986, pp. 229-32, describesa process known as "time-division gas chopping" that uses a reactiveprocess gas flow as the process activation switch. Although theseprocesses represent the most advanced known uses of single activationswitches for material layer deposition and etch processes, respectively,each of these techniques have significant limitations.

LRP provides a capability for multilayer processing by using wafertemperature cycling as the only available process activation switch.Process environment for deposition of a subsequent layer is stabilizedwhile the thermal switch is off. In LRP, a semiconductor wafer is placedwithin a single-wafer reaction process chamber filled with a desiredprocess gas (e.g. for epitaxial silicon growth or polysilicondeposition). By only using a single thermal process activation switch,usually a heating lamp, LRP can deposit multiple material layers on asemiconductor wafer. To begin the process, first the process ambient isestablished by starting the desired process gas flows while the thermalactivation switch is off (cold wafer). Then, the semiconductor wafer isheated to the desired process temperature by activating the processenergy source (heating lamp). Once the wafer reaches the processtemperature, a layer of material (e.g., epitaxial silicon) is depositedon the wafer to a desired thickness. Once the layer reaches the desiredthickness, the heat source, which is the main process energy source, isturned off. The next step is to evacuate the process gases from thereactor process chamber and to establish the process environment for thenext in-situ deposition step while the process activation switch(heating source) remains off. The semiconductor wafer is then restoredto a high temperature by activating the heating source in preparationfor a next elevated-temperature material layer growth or depositionstep. An example of an LRP sequence may be as follows: a first layer maybe an epitaxial silicon layer, a second layer may be an SiO₂ layer, athird layer may be a polysilicon layer, followed by another oxidationstep.

Because LRP uses only one process activation switch, it provides limitedflexibility in depositing multiple layers by in-situ multiprocessing.Additionally, temperature activation switching used in LRP and othermultiprocessing techniques can cause thermally-induced defects andstresses, both within the semiconductor substance and between thedeposited or grown layers. The thermally-induced stresses and defectssuch as slip dislocations can arise from rapid cooling and heating thatoccurs between fabrication process steps. Since thermal energy is theonly available process activation energy source, the processtemperatures in LRP are rather high (e.g., 650°-1200° C. ) which canfurther increase the thermally-induced defect generation problem.

Consequently, there is need for semiconductor wafer multiprocessingtechniques that permit in-situ deposition and/or etching of multiplelayers on a semiconductor substrate with more than one processactivation switch and multiple process energy sources.

There is also a need for a semiconductor wafer fabrication process thateliminates excessive thermal stresses and substrate defects caused byrepetitive semiconductor wafer heating and cooling cycles betweenmultiple layer depositions.

Time-division gas chopping eliminates some problems or limitationsassociated with conventional etching techniques. For example, someconventional plasma-assisted anisotropic etching techniques use amixture of at least two process gases within a process chamber togenerate a mixed gas plasma. The mixed process medium usually consistsof a sidewall passivation gas and an etch gas. Using a composite processmedium, however, gas discharge characteristics can become complex andmany different activated species are produced within the processchamber. Sometimes the discharge characteristics are predictable; othertimes there are interactive gas-phase reaction effects between differentactivated species that are neither easily predictable nor desireable.For example, using a combination of two or more gases in the processchamber, gas discharge can produce a composite plasma medium that yieldsunwanted gas-phase reactions and nucleations. Gas-phase nucleations arecaused by reactions between different activated plasma species,resulting in generation of particles and reduction of the semiconductordevice fabrication yield. Thus, it is preferred to avoid those possiblegas-phase reactions by reducing the number of different gases presentwithin the activated plasma environments at the same time.

Anisotropic etching processes are expected to produce well-defined andnear-vertical patterned layer sidewalls on semiconductor wafersfollowing the removal of the etched portions. These near-vertical orvertical pattern sidewalls with minimal undercut are essential to thedesign and performance of the resulting integrated circuit. Particlesfrom gas-phase nucleations within a composite plasma environment canproduce defects on semiconductor devices. However, the use of multipleprocess gases and composite plasmas are essential in some anisotropicetch processes in order to prevent lateral etching or etch undercut andto increase the degree of plasma etch anisotropy. This is due to thefact that at least one gas in the mixed plasma environment forms apassivation layer on pattern sidewalls and prevents lateral etch orundercut.

Time-division gas chopping attempts to solve these problems ofconventional plasma-assisted anisotropic etch processes by periodicallyswitching between two different process gases. The first gas is usuallya reactive etch gas (e.g. a halogen-containing compound) which canremove any exposed material layers; the second gas is a sidewallpassivation gas which is supposed to prevent pattern lateral sidewalletch. The etch gas serves to etch the exposed material layer on asemiconductor wafer according to a pre-specified pattern designtransferred to a photoresist or a hard-mask layer by microlithography.The sidewall passivation gas deposits a thin etch passivation layer onthe substrate surface including etched pattern sidewalls. The thinsidewall passivation layer prevents lateral etching or undercutting ofthe sidewalls that could occur by the etch gas during a subsequentmaterial etch step. This protects the pattern sidewalls during ananisotropic etching process following the surface passivation step. Thepassivation layer is sufficiently thin that during a subsequentanisotropic etching step, the directional and energetic ions in plasmacan remove it from flat horizontal surfaces and form the desired circuitpattern by anisotropic etching. Sequentially depositing a passivationlayer and performing an anisotropic etch step significantly reduceslateral sidewall etching. This results in an overall fabrication processyield improvement.

Time-division gas chopping also minimizes composite discharge effectsand gas-phase nucleations and particle generation by using only oneplasma process gas at a time. Using only one gas at a time also yieldsmore predictable and reproducible etch results from one semiconductorwafer to another (wafer-to-wafer process repeatability).

Although time-division gas chopping results in some improvements overconventional mixed-gas composite plasma-assisted anisotropic etchingtechniques, significant limitations also exist in that process. Bycyclicly or sequentially introducing the etch gas followed by thepassivation gas, a throughput or etch rate degradation occurs, becauseof the rather slow gas flow transient times and the need to pump out theprocess chamber after each etch or passivation cycle and to reestablishthe process environment for the subsequent passivation or etch processstep at the end of each preceding process step. This requires acompletely new volume of process gas to perform each etching orpassivation step in a multi-step anisotropic etch process.

Another problem of time-division gas flow chopping also relates to therepeated etch process gas/passivation gas cycling. By starting andstopping gas flows through gas lines connected to the fabricationreactor, particulates from the gas lines and valves may enter theprocess chamber and contaminate the semiconductor wafer. Each suddensurge of gas flow, whether opening or closing the gas valves ormass-flow controllers, can introduce particulates into the processchamber. This particulate contamination degrades the semiconductordevice manufacturing yield.

Yet another limitation associated with the time-division gas choppingtechnique relates to long gas flow transient times required forstabilizing the process chamber for sequential etch and passivationplasma environments between adjacent cycles. With each cycle, timedelays on the order of several seconds to tens of seconds may benecessary to pump out the preceding process gas ambient and to stabilizethe new process environment. For a given anisotropic etch process, itmay be necessary to use a number of etch-passivation cycles in order tocomplete the etch process. As a result, the gas on/off cycles andprocess stabilization periods may contribute significantly to the totaletch process time for each semiconductor wafer.

Thus, there is a need for a plasma-assisted semiconductor devicefabrication technique that eliminates the need for composite mixed-gasdischarge and minimizes any gas-phase nucleations and particulatecontamination without reducing the overall wafer processing throughout.

There is a need for plasma-assisted semiconductor device fabricationetch and deposition techniques that allow the use of various processgases without the transient flow surges and particulate contaminationfrom the gas flow lines and valves that may contaminate thesemiconductor wafer.

There is yet a need for a plasma-assisted device fabrication techniquethat permits the use of alternating etch gas/passivation plasmaprocessing cycles without having to deal with the long transient timesassociated with the time-division gas flow cycling method.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, there is provided amethod and apparatus for time-division plasma chopping in amulti-channel plasma processing reactor for various material layer etchand deposition applications. The invention comprises the necessary gasflow channels and methodology for the controlled simultaneous use ofmultiple process activation switches including thermal energy andradio-frequency (RF) gas discharge energy sources disposed within theprocess chamber as well as process plasma energy sources placed externalto the process chamber, to overcome the limitations associated withknown deposition and etching methods for semiconductor devicefabrication.

According to one aspect of the invention, there is provided atime-division multi-channel plasma chopping apparatus that includes amain plasma transfer channel to transfer various process gases andactivated plasma flows into a fabrication reactor wafer processingchamber. A plurality of gas discharge channels feed various process gasand plasma combinations to the main transfer channel. Each gas dischargeor activation channel has at least one energy/excitation source with itsassociated process activation switch that independently controls plasmageneration within the gas discharge channel. The process energy sourcein each gas discharge or activation channel may be a radio-frequencypower source, a microwave source, a deep ultraviolet source, or othergas excitation sources.

By selectively controlling the process energy/excitation sources ortheir associated process activation switches, the present inventionallows an operator to produce various process plasmas or activatedspecies according to the requirements of a particular fabricationprocess step. The combined effects of process plasma resulting fromselective activation of one or more of these energy/excitation sources(plasma or photochemical) and a thermal energy source and/or aradio-frequency chuck associated with the wafer, within the fabricationreactor are to start and stop different process steps in a multi-stepsemiconductor device fabrication process. This methodology results inreduced process gas cycling and enhanced processing throughout for agiven fabrication process.

Another aspect of the present invention includes a method formulti-channel time-division plasma chopping in association withplasma-assisted and/or photochemical processing in a semiconductor waferprocessing equipment. The method comprises the steps of independentlyand selectively generating a plurality of process plasmas or activatedspecies using a plurality of independent process gas excitation sourcesconfigured in parallel. Various process gases are independently directedthrough a plurality of discharge or gas activation channels associatedwith respective gas excitation sources towards a main process gas/plasmatransfer channel. The mixture of molecular gases and activated waferplasma are directed through the main fluid transfer channel to theprocess chamber of the fabrication reactor where the semiconductorsubstrate is placed. Depending on the desired step of a multi-stepchopped plasma process, the method includes independently andselectively controlling the "off" and "on" states of the processactivation switches or energy/excitation sources according to a specificsemiconductor device fabrication recipe.

Continuously flowing process gases through the individual gas activationdischarge channels and through the main transfer channel to the processchamber during a semiconductor device fabrication process is animportant aspect of the present invention. Various processenergy/excitation sources or process activation switches for themulti-channel wafer processing equipment may include deep ultravioletphotochemical, remote microwave plasma, and other plasma generating orgas activation sources.

A technical advantage of the present invention is that it overcomesthermal stress limitations associated with conventional multiprocessingtechniques such as limited reaction processing. Not only does thepresent invention permit the simultaneous use of multiple process energysources beyond only a thermal activation energy source, but alsothermally induced stresses and defects are eliminated in the presentinvention due to reduced dependency of in-situ multiprocessing recipeson cycling of thermal energy (wafer temperature) for process activationand control. This is true, because using energy sources other than onlya thermal activation source allows the use of much lower processtemperatures. Lower wafer temperatures in conjunction with theavailability of additional process activation sources (e.g., plasma,photochemical) result in reduced amount of wafer temperature cycling andthermally induced stresses in a given multi-step process.

Yet another technical advantage of the present invention is that itprovides all the advantages of time-division gas chopping in anisotropicplasma etch processes without the limitations associated with thattechnique. For example, using the time-division plasma choppingtechnique of the present invention, multiple and frequent process gasswitching is not necessary. Multi-channel plasma chopping allowscontinuous flow of several process gases with selective time-divisionchopping of plasma excitation sources in various plasma gas excitationchannels. This eliminates particulate contamination of a process chamberand semiconductor substrate due to process gas switching and improvesthe overall process throughout.

Still another technical advantage of the present invention is thatunlike the gas flow chopping method, there is no need to use repeatedsequential starting and stopping (or switching) of process gases, i.e.,etch gas followed by passivation gas, through the process reactor. Thisflexibility eliminates the long gas flow transient times and also thelong process ambient stabilization times associated with time-divisiongas chopping. As a result, the time-division multi-channel plasmachopping technique of the present invention significantly improvesprocess throughput relative to conventional time-division gas chopping.

A further technical advantage of the time division plasma choppingdevice and method of the present invention is that it permits the use ofnumerous process activation switches and energy sources in parallel toperform sequential multi-step semiconductor device processing.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, as well as modes of use and further advantages, is bestunderstood by reference to the following description of illustrativeembodiments when read in conjunction with the accompanying drawings.

FIG. 1 shows a schematic block diagram to illustrate fundamentalconcepts of multi-switch processing and the multi-channel time-divisionenergy source chopping apparatus of the present invention;

FIG. 2 is a time-sequence chart or process switch pulse diagram of amulti-cycle semiconductor fabrication process sequence employing themulti-switch processing apparatus and method of the present inventionfor in-situ epitaxial layer multiprocessing;

FIG. 3 shows a partially-broken-away diagrammatic view of amulti-channel plasma processing system with the remote microwave plasmachannels employing a preferred embodiment of the multi-channeltime-division plasma chopping apparatus of the present invention;

FIG. 4 illustrates one embodiment of the multi-channel gas dischargemodule, of the present invention comprising a plurality of (four)stacked, independent remote plasma process energy sources and theirassociated microwave plasma gas discharge activating switches accordingto the present invention;

FIG. 5 shows an alternative embodiment of the multi-channel gasdischarge module of the present invention incorporating multiple (four)radially-distributed (or cylindrically-distributed) microwave plasmaprocess energy sources and their associated process activation switches;

FIG. 6 illustrates the time sequence or pulse diagram of processactivation switch control signals of multiple, independent plasmaprocess energy sources (two remote microwave and one RF plasma)according to one embodiment of the present invention to perform aplasma-assisted semiconductor wafer anisotropic etching process; and

FIG. 7 shows the time sequence or pulse diagram of process activationswitch control signals of multiple, independent plasma process energysources for a multi-channel time-division plasma chopping processaccording to the present invention for a plasma-assisted semiconductorwafer in-situ multi-layer deposition process.

DETAILED DESCRIPTION OF THE INVENTION

The preferred embodiment of the present invention is best understood byreferring to the FIGUREs, like numerals being used for like andcorresponding parts of the various drawings.

FIG. 1 is a schematic, block-diagram illustrating multi-switchprocessing concepts fundamental to the multi-channel time-divisionplasma chopping apparatus and method 10 of the present invention.According to FIG. 1, the multi-switch time-division processing apparatus10 of the present invention comprises multiple (two or more)independently-operated process energy/excitation sources 16, 20, 24, 28and 32 and process gas sources 36, 38, 40, 42 and 44 which associatewith semiconductor wafer 12 within process chamber 14. Processenergy/excitation source-S1 16 associates via energy source coupler 18with process chamber 14. Similarly, energy/excitation sources S2 through-Sn 20, 24, 28 and 32 associate with process chamber 14 via energysource couplers 22, 26, 30 and 34. Input gas channels G₁ through Gm 36,38, 40, 42 and 44, respectively, provide process gases into processchamber 14. Each process/activation source may either act directly onthe wafer and entire process environment (mixture of gases) within theprocess chamber or may selectively and remotely activate a pre-specifiedgas channel.

The multi-switch processing equipment configuration of FIG. 1 has usefor thin film deposition, material layer etching, or semiconductor wafercleaning and annealing. One respect in which the conceptualconfiguration of FIG. 1 differs from a conventional processing equipmentconfiguration is that the only activation switch for conventionalprocesses is usually a single process energy source such as RF plasma orwafer thermal energy. By using multiple independently-controlled processenergy sources and their associated activation switches to providemulti-switch processing and/or time-division plasma chopping, thepresent invention avoids problems associated with both excessivetemperature cycling and gas flow cycling of known processing techniques.For example, in conventional processes such as limited reactionprocessing ("LRP"), large-range wafer temperature cycling occurs foreach stage of a multi-step process.

Using multiple process activation switches and energy sources, thepresent invention can achieve the same or better results than LRP withreduced degree of thermal cycling and without the associatedsemiconductor wafer thermal stresses and slip dislocations. Anotheradvantage of the present invention is that it permits the simultaneoususe of two or more independent process energy sources by proper controlof process activation switches in the time domain. Process activationswitches for the present invention could associate with various processenergy sources such as wafer temperature elevation by a heating lamp orresistive heat source, radio-frequency plasma generation within theprocess chamber, and remote microwave plasma generation by one or moreof the remote excitation sources (S1 through Sn represent the processactivation switches for "n" process energy sources). Additionally, S1through Sn may control any combination of photochemical sources, deepultraviolet coherent and incoherent sources, or other process energysources capable of operating as process and gas activation elements. Theprocess activation switches may be operated as digital or binary ON/OFFswitches. The "ON" state of a given switch will activate its associatedprocess energy source at a predetermined energy level. The " OFF" stateof any process activation switch removes (or inactivates) the processenergy source associated with the off-state switch. On the other hand,any of the process activation switches may operate its associatedprocess energy source over more than two discrete levels. These multipleswitch levels could correspond to one "OFF" state and several "ON"states to deliver different energy fractions of a process energy sourceto the process environment. In general, any process activation switchmay be a binary ON/OFF switch or a multi-level digitized switch with one"OFF" and multiple "ON" states. Analog switches may also be used whichwill allow operation of energy source over a continuum. The multi-switchprocessing and time-division plasma chopping methods and apparatus ofthis invention can employ various types of process activation switches.The "ON" state level of any process energy source is usually optimizedfor a given process and its choice may also be affected by the presenceof other process energy sources activated during a given processingstep.

FIG. 2 illustrates the use of five independent process energy sourcesand their associated activation switches that may take the configurationthat FIG. 1 depicts. Along the left-hand vertical side of FIG. 2 appearsa listing of various process activation switches that may be used in amulti-step plasma-assisted device fabrication process. Switch "S1" is awafer heating or thermal energy source such as a heating lamp module;Switch "S2", an ECR or electron cyclotron resonance microwave plasmasource; S3, a deep ultraviolet incoherent photochemical source; S4, adeep ultraviolet excimer laser photochemical source; and S5, an RFmagnetron plasma source. Each Switch S1 through S5 has a time line torepresent the pulse diagram time sequence from the beginning of themulti-step fabrication process. The time lines move from the leftvertical side (process start) of FIG. 2 to the right vertical side(process end). Elevated portions or high states of each signal indicatetimes when the respective process activation switch operates in anenergized or excited state.

The bottom of FIG. 2 shows, from left to right, a sequence of severalprocess steps for performing a multi-step in-situ semiconductor devicefabrication process. The steps in this representative example may begrouped into nine sub-processes as follows:

1. Pre-epitaxial dry cleaning to remove organic contaminants;

2. Pre-epitaxial dry cleaning to remove metallic contaminants;

3. Pre-epitaxial dry cleaning to remove native oxide;

4. Low-temperature silicon epitaxial growth;

5. Germanium/silicon epitaxial growth;

6. Low-temperature silicon epitaxial growth;

7. Gate oxidation;

8. Polysilicon layer deposition; and

9. Polysilicon layer doping.

Five independent process energy sources (and process activationswitches) are used in this example. The precise number of necessaryprocess energy sources and their types can vary depending on themulti-channel processing equipment design and the in-situmulti-processing requirements. The process energy sources shown in thisexample can be sequentially controlled in the time domain such that theentire nine sub-process steps are performed with reduced thermal cyclingor switching of the thermal energy source. All these process activationsources can be implemented in a properly designed multi-processingequipment consisting of ECR plasma source, wafer heating source, RFmagnetron plasma, and photochemical energy sources.

Each of the process activation switches S1 through S5 can be operatedindependently to perform its respective process energy source controlfor the multi-step fabrication process. The multi-step in-situfabrication process example of FIG. 2 takes place according to thefollowing sequence. First, the process chamber 14 is vacuum-pumped atstep T0. This establishes the necessary initial clean environment toperform the fabrication process sequence. Next, the pre-epitaxial drycleaning of organics occurs. This sub-process established the necessaryprocess environment during step T1 when all the process activationswitches are off. This sub-process uses Switch S3 to activate the deepultraviolet incoherent photochemical source to activate the organiccleaning process during step T2. The high or raised portion of SwitchS3's time line corresponding to step T2 indicates energizing processactivation Switch S3. Following the pre-epitaxial dry cleaning processof step T2, the processing chamber is again pumped down to vacuum atstep T3.

Pre-epitaxial dry removal of metals, the next sub-process, uses as theprocess gas a mixture of Ar and HCl gases. The process environment forthis sub-process is stabilized during time step T4 when all processactivation switches are off again. This process activates Switch S2-ECRmicrowave plasma process energy source and Switch S1-wafer heating lampmodule for low-temperature ECR plasma-assisted process activation.During the pre-epitaxial dry cleaning process T5, Switch S2 and SwitchS1 remain energized. Following the pre-epitaxial dry metallic cleaningprocess T5, however, Switch S2 for ECR plasma is inactivated and SwitchS1 for thermal energy remains energized. Switch S1 will remain energizedthrough a portion of the remaining steps in the multi-step fabricationprocess of FIG. 2 in order to minimize wafer thermal cycling and enhanceoverall processing throughout. The activated level of thermal Switch S1is sufficiently low such that it cannot drive a subsequent epitaxialsilicon growth process by itself unless an additional process energysource is activated. The subsequent sub-process establishes thenecessary oxide removal process environment (HF+H₂ +Ar ambient) duringtime step T7 following a vacuum pump down step T6. The native oxideremoval process step employs three simultaneous process energy sourcesby activating additional process Switches S2 (ECR plasma) and S3 (deepultraviolet light photochemical) while the thermal energy source SwitchS1 remains activated. This is done during the time step T8. Followingthe native oxide removal step T8, process activation Switches S2 and S3are turned off and a vacuum pump down step T9 occurs (while the thermalenergy Switch S1 remains on in order to minimize wafer thermal cycling).Immediately following the pre-epitaxial dry cleaning process T8, vacuumpump down T9 of process chamber 14 occurs.

Sub-process 4, a low-temperature in-situ-doped epitaxial silicon growthon the semiconductor wafer takes place next. This sub-process uses amixture of gases, including SiH₂ Cl₂, H₂, Ar, and AsH₃. The epitaxialgrowth process environment is stabilized during time step T10 when onlythermal Switch S1 is on. However, the thermal switch by itself cannotcause epitaxial silicon growth. Process activation sources includingSwitch S1-wafer heating source and Switch S2-ECR microwave switchprovide necessary process excitation energy sources for thelow-temperature epitaxial silicon growth process T11. Following thecompletion of epitaxial growth step S11, process chamber 14 vacuum pumpdown T12 occurs. At this time, Switch S1 and Switch S2 de-energize. Thisis the first time Switch S1 or wafer thermal energy has beende-energized since being initially energized for pre-epitaxial drycleaning step T5. This shows how using more than one process activationswitch eliminates the need for repeated thermal cycling that may occurwhen using only the wafer heating source as process activation switch.Moreover, temperatures that the wafer heating source produces at thesemiconductor wafer in a multi-switch processing equipment can be lowerthan those required when the wafer heating source is the only processactivation switch. As a result, the extreme high temperatures occurringin that case are eliminated with the use of multiple activation switchesand process energy sources.

In-situ-doped germanium-silicon epitaxial growth is the next set ofsteps in the multi-step fabrication process example of FIG. 2. The gascombination for this subprocess is a mixture of SiH2Cl₂, GeH₄, H₂, Arand B₂ H₆. The process ambient for this sub-process is stabilized duringtime step T13 during which all the process activation switches,including the thermal Switch S1, are off. The thermal Switch S1 is offbecause it may activate the Ge_(x) Si.sub.(1-x) growth process even byitself. This process includes germanium-silicon epitaxial growth stepT14 which uses only Switch S1-wafer heating source as the activationswitch. Next, process chamber 14 is pumped down T15 to establish processchamber conditions for the next set of fabrication steps. The processactivation Switch S1 is turned off during T15 in order to prevent anyuncontrolled epitaxial Ge_(x) Si.sub.(1-x) growth during chamber pumpdown.

Low-temperature silicon epitaxial growth using a process gas mixture ofSiH₂ Cl₂, H₂, Ar and AsH₃ is the next group of steps in the sequence ofFIG. 2. This sub-process includes the low-temperature epitaxial silicongrowth step T17. This process uses Switch S1-wafer heating source andSwitch S2-ECR microwave plasma source for low-temperature processactivation. Following completion of silicon epitaxial growth step T17,vacuum pump down occurs T18 at which time Switch S2 is de-energized, butSwitch S1 remains energized. This is because thermal energy by itselfcannot drive epitaxial silicon process at low (e.g. T≦500°C.)temperatures.

Next, gate oxidation occurs using combined oxygen and argon gases. Theoxidation step T20 uses three process energy sources, based on theactivation of Switch S1-wafer heating source which is already energized,Switch S2-ECR microwave plasma source, and. Switch S3-deep ultravioletincoherent photon source. These switches provide process activationenergy until vacuum pump down T21 occurs. At vacuum pump down T21,Switch S2 and Switch S3 are de-energized, however, Switch S1 stillremains energized to minimize thermal cycling.

Polysilicon deposition T23 takes place next using a composition of SiH₄,H₂ and Ar gases. Polysilicon deposition step T23 activated switchesinclude Switch S1 for wafer heating source and Switch S5 for RFmagnetron plasma source. Following polysilicon deposition T23, vacuumpump down T24 occurs, to prepare the process chamber for the finalfabrication step. All process energy sources are off during the vacuumpump down step T24.

In the multi-step fabrication process of the present example, the finalsub-process is polysilicon doping. During polysilicon doping, acombination of AsH₃ and H₂ gases are used to dope the silicon layerresulting from the prior polysilicon deposition step. For this purpose,Switch S4 is activated in order to operate the deep ultraviolet excimerlaser source as the process energy source. Doping step T26 achieves thedesired doping level by gas-immersion laser doping and a final vacuumpump down T27 then occurs. This completes the multi-step in-situsemiconductor device fabrication process according to the example ofFIG. 2 using the apparatus and method of the present invention. Themulti-step process of FIG. 2 can also use alternative and differentcombinations of the multiple process energy sources used in thisexample.

Although FIG. 2 illustrates a simplified example, virtually unlimitedvariations of the concept of using multiple process energy sources andactivation switches for multi-step in-situ semiconductor devicefabrication are possible and should be considered within the scope ofthe present invention. For example, although the example of FIG. 2 showsa degree of thermal cycling in energizing and deenergizing Switch S1,because the wafer temperature level that activated Switch S1 creates isless than that necessary for epitaxial silicon process activation usingthermal Switch S1 alone, less thermal cycling than in conventionaltechniques can be achieved. Alternatively, because Switch S1 can beoperated at less than the level necessary for epitaxial silicon growthprocess activation, Switch S1 could be left energized throughout severalstages of the multi-step fabrication process. This would eliminatevirtually most thermal cycling during an in-situ multi-step fabricationprocess.

Note that in the transition between individual sub-processes, vacuumpump down cycles clear process chamber of gases from the immediatelyprevious cycle. These transition times result in some delay from oneprocess step to the next step. However, reduced thermal cycling offeredby the multi-switch processing methodology of this invention cancompensate for the pump-down transition delay times. Moreover, the timedivision plasma chopping of this invention can significantly reduce thenumber of gas switching or process ambient transition cycles.

The multi-switch processing method illustrated by the example of FIG. 2shows the use of five independently controlled process activationSwitches S1 through S5. Using these five independent process activationswitches, multiple steps can be performed in a device fabricationprocess with minimal temperature cycling, and increased throughputrelative to conventional wafer processing methods. Although five processenergy sources are used in this example, the same process sequence canbe performed with fewer process activation sources.

A related and important aspect of this invention relates to atime-division plasma chopping methodology and device for in-situmulti-step processing with reduced process ambient cycling and gas flowcycling effects. FIG. 3 illustrates an application of a preferredembodiment of the multi-channel time-division plasma chopping apparatusof the present invention within a single-wafer plasma processingreactor. As will be described in detail, FIG. 3 shows the use of threeindependent plasma process activation sources. It may be beneficial,however, to appreciate a typical wafer processing environment for theiruse.

Beginning at the bottom right corner of FIG. 3, the semiconductor waferprocessing system 10 includes gas distribution network 50. From gasdistribution network 50 come microwave plasma gas inlet-1 52 andmicrowave plasma gas inlet-2 54. Microwave plasma gas inlets 52 and 54connect via gas discharge tube connections 56 and 58 to microwavedischarge tubes 18 and 22 respectively. Microwave discharge cavities 60and 62 can generate gas discharge and send remotely generated plasmastreams into discharge tubes 18 and 22. Discharge tubes 18 and 22 mergeinto a main gas transfer channel 72. Main transfer channel 72 penetratesreactor casing 74 and process chamber 14 to lead through support plate76. Main transfer channel 72 connects to plasma injector 78 withinquartz or metallic cylinder 82. Also, leading from gas distributionnetwork 50, input gas channel 36 penetrates through reactor casing 74,into process chamber 14, through support plate 76, and into gas injector80 surrounded by quartz or metallic cylinder 82. The single-wafer plasmaprocessing reactor of FIG. 3 employs two remote microwave plasma processenergy sources besides an RF source on the process chamber. Ifnecessary, the number of parallel microwave plasma channels can beincreased beyond two. For plasma generation, microwave source 16 andmicrowave source 20 connect to microwave discharge cavities 60 and 62,respectively. Above quartz or metallic cylinder 82, sits semiconductorwafer 12 clamped against the RF chuck surface (with wafer heating and/orcooling capability) and supported by holding pins 84 which attach tosupport plate 76. Load lock chamber 90, within plasma processing system10, includes isolation gate 32 that comprises a passage for waferhandling robot 94 to transfer semiconductor wafers between thefabrication position on pins 84 in process chamber 14 and wafer cassette96. To maintain vacuum in both process chamber 14 and load lock chamber90, pumping package 98 connects via vacuum apertures 100 and 102.

Through RF chuck surface 86, multi-purpose chuck 88 provides RF processactivation energy for plasma processing of semiconductor wafer 12clamped against the chuck. Process control computer 104 controlsoperation of heating (or cooling) power supply 106 and chuck module RFpower supply and tuning circuit 108 to control application of RFelectromagnetic energy to multipurpose chuck 88. Process controlcomputer 104 also controls process plasma generation by controlling theoperation of microwave sources 16 and 20 (microwave switch control).

Although the configuration of FIG. 3 has fewer process activationswitches than the conceptual illustration of FIG. 1, it illustrates theuse of two parallel remote plasma process energy sources in connectionwith a multipurpose RF chuck for semiconductor device fabrication. Theconfiguration of FIG. 3 may, alternatively, include deep ultravioletphotochemical process energy sources instead of microwave plasma sourcesfor process activation, or alternatively may include a thermal heatingsource such as the lamp heating module of Switch S1 in FIG. 2. Theessence of FIG. 3, however, is that multiple independent processactivation sources can be used to significantly improve equipmentperformance and processing throughput within the conventional design ofa semiconductor wafer plasma processing system 10.

FIGS. 4 and 5 show alternative implementations of the parallel remoteplasma generation sources of FIG. 3 when more remote plasma channels arerequired. In FIG. 4, gases 1, 2, 3, and 4 enter plasma gas inlets 52,54,110 and 112, respectively. These lead to respective cavity dischargetube connections 56, 58, 114 and 116 which pass through microwavecavities 1 through 4, 60, 62,118 and 120. Associated with each microwavecavity 1 through 4 are magnetron microwave power sources M1 through M4(16, 20, 24, and 28, respectively). Control signals 122, 124,126, and128, respectively, come from a process control computer to control thesemagnetron sources. These control signals act as process activationswitches. Respective outlets from microwave cavities 1 through 4, (18,22, 26 and 30, respectively) merge at main transfer channel 72 whichpermits continuous process gas flow to process chamber 14. FIG. 5 shows,instead of the stacked configuration of FIG. 4, a radial or cylindricalconfiguration in which remote plasma tubes 18, 22, 26, and 30 radiallyconnect to main transfer channel 72. Other configurations are alsopossible.

These multi-channel plasma devices are essential components of thetime-division plasma chopping of this invention. The parallel remoteplasma channels may employ RF induction discharge energy sources insteadof microwave cavities.

An example of using the independent remote plasma generation sources,would be where gas 1 is nitrogen flowing through microwave cavity 1 60,gas 2 would be nitrous oxide or oxygen flowing through microwave cavity2 62, gas 3 at cavity 3 would be argon, and gas 4 through cavity 4 wouldbe silane or dichlorasilane gas. By controlling, not the gas flow as intime-division gas flow chopping, but control signals 122, 124, 126 and128, continuous gas flows from the plasma chopping device of the presentinvention are possible (all gases flowing), while each gas can beselectively activated according to a timing sequence to drive a desiredand specific deposition process associated with the activated plasmachannel or channels.

The inactivated gas channels allow continuous molecular gas flows andwill have little or negligible effect on the overall process. Thiseliminates excessive gas flow cycling and process ambient stabilizationtransients and associated particulate contamination problem of repeatedgas cycling. Thus, to generate a plasma, magnetron microwave source M1can be operated variably among several states or between two "ON" and"OFF" states that generate plasma from gas 1 within discharge cavity 1.The only activated plasma entering the process chamber, then, will bethat from microwave discharge cavity 1. Depending on the application,one or more microwave channels are activated during any process step todrive a desired process at any time.

FIG. 6 shows use of the time-division plasma chopping technique of thepresent invention with two remote microwave gas discharge and one RFplasma channel to achieve the multi-step sequential etching-passivationlayer deposition and perform a net anisotropic tungsten etch process.According to FIG. 6, activation Switch S1 intermittently controlsmagnetron source M1. A process etch gas such as SF₆ flows continuouslythrough this plasma channel. Activation Switch S2 controls magnetronmicrowave switch M2 and activation Switch S3 is a multi-purpose RF chuckin a process chamber (additional RF plasma activation). The process ofFIG. 6 entails injecting two process gases continuously through remotemicrowave (or RF induction) discharge tubes associated with processactivation switches M1 and M2. Plasma discharge can also occur or besubstantially enhanced by direct interaction of the remote process andplasma gases with an RF chuck within the main process chamber. Themicrowave plasma activation Switches S1 and S2, and RF chuck activationSwitch S3 and any substrate heating or cooling energy source can beswitched on and off.

In etching processes with combined radio-frequency and remote microwaveprocess plasma activation the hybrid plasma etch rate from both the RFand microwave discharges is usually much higher than the individual etchrates with only microwave plasma energy source or radio-frequencydischarge and even much greater than the sum of the individual etchrates obtained with any of the two energy sources. Therefore, theradio-frequency plasma channel and the two remote microwave plasmachannels can be switched and cycled in time such that a high rate ofanisotropic etching proceeds in the sequence described by FIG. 6 withnegligible lateral etch or undercut. Note that according to the presentinvention, all gases can flow continuously and the time-division plasmachopping technique provides the same fabrication process improvements ofgas flow chopping without flow surges and related contamination of thesemiconductor wafer and processing throughput degradations.

As an example of the application of the present invention shown in FIG.6, suppose that Gas 1 is sulphur hexafluoride (SF₆) for tungsten etchand Gas 2 is ammonia (NH₃) used for tungsten nitride passivation layerformation. Sequential etching and passivation layer deposition occurs inthis multicycle process. In FIG. 6, starting the process begins with anetching step. During the first etch step, the process activationSwitches S1 (for SF₆ microwave discharge) and S3 (for RF plasmaenhancement) are turned on while the Switch S2 (related to NH₃ gas)remains off. As a result, a mixed RF and microwave (M₁) plasma dischargeand process enhancements act on the etch gas (SF₆) and result in a nettungsten etch.

Note that since SF₆ discharge switch S2 is off, NH₃ gas is not activatedby the microwave source and molecular NH₃ gas enters the process ambientwithout any remote gas discharge in that microwave discharge channel.The RF activation effect is rather weak or negligible on the NH₃ gas inthe process chamber since the on-state level of RF power is selected tobe fairly low. As a result, NH₃ has weak or negligible interactions withthe excited etch species and also it does not result in any appreciablepassivation layer formation during that etch cycle. On the other hand,the enhancement effects of the RF energy source on SF₆ plasma speciesare quite strong because a remote SF₆ discharge has already been formedby activating the first microwave discharge channel. It can be concludedthat the RF process energy source has significant enhancement effects onthe remote microwave plasma species while its effects on molecularprocess gases without any remote excitation is rather weak ornegligible.

It can be seen that in this example, the RF energy source has two "ON"states. During the first step, the radio-frequency channel is turned onand for a short period of time the RF power level is kept at a higherthan normal level (higher "on" state) in order to remove any passivationor contamination layer from the flat horizontal surfaces by directionalenergetic ions without removing it from the side walls. Then, the etchproceeds with minimal surface and gas-phase nucleation effects of theammonia molecules. During the following side-wall passivation cycle, NH₃remote plasma forms a tungsten WN_(x) passivant layer on thesemiconductor wafer (on horizontal as well as sidewall surfaces). Forthis step, microwave channel M2 is turned on, and both microwave channelM1 and RF channel are turned off. This configuration produces a thintungsten nitride passivant layer by an isotropic reaction on thesemiconductor wafer surface without generating any appreciable amount offluorine-activated etch species, even though both SF₆ and NH₃ arecontinually flowing into the main process chamber. The subsequent etchand passivation steps follow the first two steps in a periodic manneruntil the tungsten layer etch is completed. The number ofetch/passivation cycles employed in a given etch process can varydepending on the specific etch process requirements. A typicalsingle-wafer etch process may employ 3-100 etch passivation cycles.Although the example in FIG. 6 shows the application of time-divisionplasma chopping of this invention to anisotropic tungsten etch using SF₆/HN₃ gas flows in the multi-channel plasma processing reactor, thisinvention is applicable to a wide range of other etch and depositionapplications based on different process chemistries. For instance, thisinvention can be used for high throughput etching of stacked multilayerstructures where each layer requires a different activated etch species(e.g. polycide gates). This can be done by selective time-divisionplasma chopping techniques of this invention while all or some processgases are continuously flowing.

FIG. 7 shows yet another application of the time-division plasmachopping techniques of the present invention for multilayer dielectricand semiconductor deposition. FIG. 7 shows, for example, the use of tworemote process activation switches associated with two parallelmicrowave gas discharge plasma channels. In conjunction with the thermalactivation Switch S4 and an RF plasma activation, Switch S3, theapplication of FIG. 1 uses two separate remote microwave plasma energysource chopping channels (S1 and S2). The process in FIG. 1 yields adielectric and amorphous silicon superlattice heterostructure. Thisin-situ multi-step process can be performed in the multi-channel plasmaprocessing reactor of FIG. 3.

Suppose that Gas 1 is N₂ O. Gas 2 is N₂, both passing through remotemicrowave plasma channels M1 and M2. A third process gas introduceddirectly into the main process chamber may be a silicon source gas suchas silane (SiH₄), or dichlorosilane (SiH₂ Cl₂). The process of FIG. 7starts with all process activation switches (S1-S4) de-energized andcontinuously flowing all process gases into the process chamber. Thiswill stabilize the overall process environment with all the processenergy sources in the "OFF" state.

For the first layer, energizing activation Switch S1 results inselective N₂ O gas discharged and deposition of silicon dioxide (SiO₂)layer. During deposition of the first layer, activation Switch S1provides microwave plasma activation of N₂ O by turning M1 from anoff-state to an on-state. The RF plasma activation Switch S3 is alsoactivated and remains on during all deposition steps. The first andsubsequent deposition steps all employ some thermal process activationby turning on Switch S4. However, Switch S4 or thermal activation byitself cannot result in any layer deposition unless some remote plasmaactivation is present. During the remainder of the steps of creating thenine-layer dielectric superlattice heterostructure of the FIG. 7example, process energy sources S3 and S4 remain energized.

Once the process reaches the desired thickness of oxide layer, a secondlayer of Si_(x) O_(y) N_(Z) is applied. To deposit the second layer, thethermal activation Switch S4, RF plasma S3, and microwave plasmachannels M1 and M2 for process gases N₂ and N₂ O are all energized. Inthe following a layer of Si₃ N₄ is deposited by de-energizing microwaveactivation Switch S1. Layer four is a thin amorphous silicon layer thatis deposited by additionally turning off microwave activation Switch S2(with only RF plasma and thermal process activation). Layer five of SiO₂is applied by energizing N₂) plasma activation Switch S1. Both RF andthermal process energy sources (S3, S4) remain activated. Next, a sixthlayer (Si_(x) O_(y) N_(z)) is applied by activating all process energysources.

A seventh layer Si₃ N₄ is deposited by deenergizing microwave plasmaactivation Switch S1 and leaving energized all other process activationsources (S2, S3, S4 on). To generate eighth thin layer of material(amorphous silicon), only the thermal and RF process energy sources areenergized and S3 and S4 remain on.

Finally, by energizing microwave plasma activation Switch S1, theprocess that FIG. 7 describes produces an SiO₂ layer. Stopping theprocess involves deenergizing thermal activation Switch S4, remoteplasma activation Switches S1 and S2, and turning off RF plasma processactivation Switch S4. FIG. 7, therefore, describes an in-situ multi-stepfabrication process that produces a nine-layer dielectric and siliconsuperlattice heterostructure without the gas cycling of gas choppingmethod by selectively energizing and using several process energysources according to a pre-specified time sequence.

It should be realized that the examples of FIGS. 6 and 7 are only two ofa virtually unlimited number of applications of the multi-switchprocessing and time-division plasma chopping techniques embodied withinthe scope of the present invention. Furthermore, although the inventionhas been described with reference to the above specific embodiments,this description is not meant to be construed in a limiting sense.Various modifications of the disclosed embodiment, as well asalternative embodiments of the invention will become apparent to personsskilled in the art upon reference to the above description it istherefore contemplated that the appended claims will cover suchmodifications that fall within the true scope of the invention.

What is claimed is:
 1. A method for multi-channel time-division plasmachopping in association with a plasma processing reactor, comprising thesteps of:independently and selectively generating a plurality of processplasmas and activated species using a plurality of independent processenergy/excitation sources, said process energy/excitation sourcesoperable to generate energy at more than two discrete levels;independently directing to a main transfer channel mixtures of processgases and said generated process plasmas and activated species through aplurality of discharge channels associated with said processenergy/excitation sources; and transferring said mixtures from said maintransfer channel to said reactor process chamber where wafer is placed.2. The method of claim 1, further comprising the step of independentlyand selectively controlling the "on" and "off" states of said excitationsources via their associated process activation switches.
 3. The methodof claim 1, further including the step of exciting said gas and plasmamixture within the plasma processing reactor using a radio-frequencyprocess energy source.
 4. The method of claim 1, wherein said mixturedirecting step further including the step of continuously flowing gasesthrough said gas discharge and activating channels to said main transferchannel.
 5. The method of claim 1, further including the step ofsequentially and selectively operating a plurality of process activationswitch controls to perform a sequential multi-step in-situ semiconductordevice fabrication process.
 6. The method of claim 1, wherein saidprocess activation step further includes the step of generating saidactivated process gas using a photochemical source.
 7. The method ofclaim 1, wherein said process activation step further includes the stepof generating said process activation using a deep ultraviolet source.8. The method of claim 1 wherein said process energy/excitation sourcesgenerate energy at three discrete levels.
 9. The method of claim 1wherein said process energy/excitation sources generate energy at acontinuum of energy levels.
 10. A method for multi-channel time-divisionplasma chopping in association with a plasma processing reactor,comprising the steps of:independently and selectively generating aplurality of process plasmas and activated species using a plurality ofindependent process energy/excitation sources; exciting said gas andplasma mixture within the plasma processing reactor using aradio-frequency process energy source; independently directing to a maintransfer channel mixtures of process gases and said generated processplasmas and activated species through a plurality of discharge channelsassociated with said process energy/excitation sources; and transferringsaid mixtures from said main transfer channel to said reactor processchamber where wafer is placed.
 11. A method for multi-channeltime-division plasma chopping in association with a plasma processingreactor, comprising the steps of:independently and selectivelygenerating a plurality of process plasmas and activated species using aplurality of independent process energy/excitation sources, at least oneof said sources being a photochemical source; independently directing toa main transfer channel mixtures of process gases and said generatedprocess plasmas and activated species through a plurality of dischargechannels associated with said process energy/excitation sources; andtransferring said mixtures from said main transfer channel to saidreactor process chamber where wafer is placed.
 12. A method formulti-channel time-division plasma chopping in association with a plasmaprocessing reactor, comprising the steps of:independently andselectively generating a plurality of process plasmas and activatedspecies using a plurality of independent process energy/excitationsources, at least one of said sources being a deep ultraviolet source;independently directing to a main transfer channel mixtures of processgases and said generated process plasmas and activated species through aplurality of discharge channels associated with said processenergy/excitation sources; and transferring said mixtures from said maintransfer channel to said reactor process chamber where wafer is placed.